In an engine, parallel and spaced apart thrust faces of a crankshaft act to axially locate the crankshaft within an engine block. During vehicle operation, in which many transmission upshifts and downshifts are made and in which the engine crankshaft can rotate at relatively high speeds, the thrust faces thereof come into contact with the thrust face bearings of the engine block. The frictional contact requires the thrust faces to have very smooth surfaces free from distortion, or runout, for smooth operation and for optimizing wear characteristics between these parts.
Prior to the present invention, burnishing or roller finishing equipment and procedures have been employed to provide highly finished wear surfaces. Generally such burnishing is drawn to a surface finishing operation for plastically deformable metal parts in which the metal is cold worked but is not removed. Such plastic deformation is completed when the roughness profile of the surface is eliminated or reduced and a desired smoothness has been achieved. Various machining operations often proceed the burnishing operation at the same station.
Before this invention, the crankshaft thrust faces have been roller burnished during the manufacturing process to materially improve the mechanical properties of the thrust face surfaces. Commonly used crankshaft roller burnishing equipment and methods include the mounting of a crankshaft to be worked between a pair of special workpiece holders. The workpiece holders arc configured in such a way as to permit lateral movement of the crankshaft (axially) to a limited extent during rotation. The crankshaft is rotated at a predetermined speed and the roller burnishing tool is advanced from a retracted position to a position between the crankshaft thrust faces. The burnishing tool is turned in a limited arc in a first direction until burnishing rollers thereof physically engage opposing thrust faces at a predetermined torque. The burnishing tool is held in this load position for a period of time to mechanically work and plastically smooth the thrust face surfaces. The burnishing tool is then turned out of engagement with the crankshaft thrust faces and returned to its retracted position.
With prior burnishing processes, the burnishing tool often does not engage each of the opposing thrust faces simultaneously as desired for even distribution of the burnishing loads but rather engages one thrust face before it engages the other. When this occurs, the force imparted by the burnishing tool is concentrated on the first thrust face that the tool contacts. The first thrust face tends to bend and distort under the increased applied burnishing load, which would be up to double the force or load than if both thrust faces were contacted simultaneously to evenly distribute the load. Such distortion of the thrust faces results in crankshaft rejection causing scrapping of such crankshafts for recycling.
Distortion of the crankshaft thrust faces crankshaft has been avoided by allowing the crankshaft to float axially relatively to the burnishing tool during the burnishing operation. This has been accomplished by employing centering springs on the crankshaft holding and turning unit so that the crank shaft can move axially relative to the burnishing tool when the tool is operatively positioned between and engages with the thrust faces whereby the tool forces such centering. With such a spring biased workpiece positioning arrangement, the relative movement between the tool and thrust faces results in the centering of the tool between the thrust faces and the simultaneous burnishing of the opposing thrust faces.
However, manufacturing situations arise in which it is not desirable or not possible to allow such lateral or axial movement of the crankshaft. One example of such a situation is one in which the crankshaft cannot be run on a production machine. This can occur when small test runs are conducted on a lathe. This can also occur when the crankshaft thrust faces become damaged during the manufacturing process and, as a result, must be either scrapped or further machined to a dimension above production tolerance. Instead of setting up the production burnishing machine to this larger dimension to rework such damaged crankshafts, which is time consuming, it is generally preferable and more efficient to burnish these crankshafts off line on a lathe.
However, burnishing crankshafts in a lathe or other machine in which the crankshaft cannot float has presented the problem of distorting the thrust faces in situations where the burnishing tool contacts one thrust face before the other. Additionally, it is desirous in some circumstances to have a production configuration in which the crankshaft does not float laterally during burnishing. Since allowing the crankshaft to float axially or laterally relative to the burnishing tool is the only known practical means to prevent distortion of the thrust faces, distortion has remained a problem with such laterally fixed burnishing apparati and methods. Hence, there is a continuing need to provide a method and apparatus in which the burnishing tool is able to move laterally relative to the crankshaft thrust faces to prevent distortion of the thrust faces and provide a finely finished part.